Facsimile signals of graphic material such as weather maps are now being printed with a thermal printing head having a linear array of, for example, 1024 minute thermal printing elements extending across the heat sensitive paper on which it will mark a line of picture elements. Such elements are called pixels, a term also identifying the elements of the original graphic matter and the corresponding electrical elements of the facsimile signal. As the paper is fed past the head it marks the paper line by line, each line comprising a series of dots at positions along the line, or the absence thereof. A printing element which receives marking current will mark a black pixel; other elements receiving no current will make no mark and leave the paper white.
While a recording having only black and white pixels is useful for many kinds of graphic material, it is often highly desirable to print pixels of gray tones or densities between black and white. It is, however, very difficult if not imposssible to supply different values of marking current to different printing elements at the same time, and thermal heads have not been used successfully hitherto for making recordings with a scale of gray tones from white to black.
Accordingly it is the object of the present to provide a method of recording facsimile recordings with a thermal print head and having a nearly continuous tone shading from black to white.